Galaxies

Galaxies

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Galaxies are the basic building blocks of the universe. Even in a tiny patch of sky, many hundreds can be seen. By surveying such regions, astronomers estimate that there may be 100 billion galaxies in the observable universe. These galaxies are not uniformly spread through space — they commonly occur in pairs, groups, clusters, and superclusters.

The classification of galaxies by shape relies upon images made with visible light. But galaxies observed in other wavelengths, or with longer exposure times, or at earlier cosmic epochs, show the need for additional categories.